Improve derive(Event) and simplify macro code

[Bleachfuel]

Objective

simplify some code and improve Event macro

Showcase

you can now write derive Events like so

#[derive(event)]
#[event(auto_propagate, traversal = MyType)]
struct MyEvent;

[Bleachfuel]

Test that is failing to compile passes on my machine, but on CI it has a weird error :(

[Bleachfuel]

Ready for review!

[Bleachfuel]

According to Dtolnay: dtolnay/syn#1850, this is not desired behaviour.

#[relationship(relationship_target = Attachments<T>)]

Instead it should be

#[relationship(relationship_target = Attachments::<T>)]

I could make it this way to make the relationship parsing nicer, but it makes it harder to understand the error for users as the error is add <T> and not add ::<T>.